[Bug 283729] sysrc destroys rc.conf if given easily-typo'd bad input

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=283729

--- Comment #1 from Bill Cole <freebsdbugz-20241229@billmail.scconsult.com> ---
Accidentally embedding a linefeed in a variable setting crashes awk and leaves
/etc/rc.conf with only the (BAD) new setting. 

Here's a transcript example: 

   [root@shinyghost ~]# sysrc -a
   clamav_clamd_enable: YES
   clamav_freshclam_enable: YES
   dovecot_enable: YES
   firewall_enable: YES
   firewall_type: /etc/ipfw.rules
   growfs_enable: YES
   ifconfig_em0: inet 192.168.254.17 netmask 255.255.255.0
   ifconfig_em0_alias0: inet 192.168.254.22 netmask 255.255.255.255
   ifconfig_em0_alias1: inet 192.168.254.26 netmask 255.255.255.255
   ifconfig_em0_alias2: inet 192.168.254.72 netmask 255.255.255.255
   ifconfig_em0_alias3: inet 192.168.254.100 netmask 255.255.255.255
   ifconfig_em0_alias4: inet 192.168.254.111 netmask 255.255.255.255
   ipfw_enable: YES
   named_enable: YES
   nfsv4_server_enable: YES
   postfix_enable: YES
   sshd_enable: YES
   [root@shinyghost ~]# sysrc nfsv4_server_enable="YES
   "
   awk: newline in string YES
   ... at source line 1t
   nfsv4_server_enable: YES -> YES
   [root@shinyghost ~]# sysrc -a
   nfsv4_server_enable: YES
   [root@shinyghost ~]# cat /etc/rc.conf
   nfsv4_server_enable="YES
   "

I know that what I'd do is replace any linefeeds in a setting value with spaces
on the input side, and on the output side do all writing of the new rc.conf to
a tempfile and then do size sanity checking of it before renaming it into
place. As a nice side-effect, the change of rc.conf would then be atomic.

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